Her diet should consist of a good quality lab block (Harlan Tech 2014 or 2018, Oxbow Regal rat, or Living World/Hagen hamster extrusian block) always available in a dish, daily vegs, supplemented several times a day with baby cereal mixed up with soy infant formula, ensure or water (without chlorine or fluoride), a bit of boiled egg, and other high protien foods.
As she weans the babies they will start eating the baby cereal, etc. They will need to be supplemented with baby cereal .... many people do it until they are two or 3 months old. Depending on the lab block you are feeding, you may want to make it into a mush for babies and mom, in addition to feeding the solid block. Babies will need a shallow water dish as they will not be srong enough to get water out of a water bottle for a long time, although they may look like they can.
Baby boys will need to be separated from sisters at 5 weeks of age so no more pregnancies. Depending on your rats, baby boys may not be able to be introduced in with their dad until 9 or 10 weeks old. That way they will be old enough to defend themselves. Intros need to be gradual. Sometimes adult rats view babies as prey and will kill them.
You don't need to wash rats. They are like cats and keep themselves clean. Some people will wash a male rat every few months because of the buck grease. Sometimes elderly rats will need help with bathing. Can't think of any other reason to bath a rat. Bathing rats isn't good for them.
If you plan on keeping both sexes, you will need to have one sex neutered or spayed. Also, if wild rats can get into your barn they could get the girls pregnant or attack the boys. Rats are usually kept in the busiest room in the house so that they receive a lot of interaction with people.
I am sure you are aware that you need to be handling the babies daily to socialize them.
The bar spacing on ferret cages will be too large for baby rats, young rats and some girls. Suggest you purchase critter nation cages.